It took weeks of walking on raggedy flip-flops and crusty bare feet, over wooded mountains and across muddy plains, before news of the atrocities could reach the outside world. Stumbling over the border into South Sudan, the refugees told of a scorched-earth offensive by the Khartoum government in Blue Nile state, a remote rebel-held area in Sudan. The refugees described months of terror in the war zone, culminating in a government offensive in May in which the Sudanese army stormed into their villages, torched homes, destroyed fields, poisoned their wells and killed whoever did not escape. Resting under a tree, Hassan Musa sat tall and taut as he described the attack on his hometown, Kukur. “They came in 25 cars,” he said. “They burned everything: the sesame fields, the peanuts, the sorghum, the homes. They killed many people. Some they shot to death, others they beat and hacked. The elderly, they burned in their houses.”

Journalists are banned from visiting Blue Nile and it is impossible to verify such accounts first hand. But the credibility of the refugees’ descriptions are strengthened by their consistency and the sheer size of the exodus: by early June, 40,000 had suddenly amassed together around the border town of El Foj. TIME spent a day at one refugee transit point known as Kilo 18 and gathered eight in-depth accounts, then compared those accounts with eight more testimonies recorded by an aid organization on the ground. The Dutch branch of Doctors Without Borders (known internationally by its French initials, MSF) has been providing medical care and water for the refugees and says the situation in Blue Nile appears “very serious.” “We are not there, so we can’t verify,” said Arjan Hehenkamp, the head of MSF-Holland. But he added that his staff has heard repeated accounts of “homes being burnt, sometimes with people inside.”

Few governments in the world have worse human rights records than Sudan’s under the leadership of President Omar al-Bashir. The refugees pouring in from Blue Nile describe tactics that have been employed repeatedly and systematically by the Sudanese regime during Bashir’s 23 years in power. In the long war between north and south Sudan, northern militias frequently raped and pillaged their way through southern territory, emptying the land of inhabitants. Similar tactics were used in Darfur. The attacks in Blue Nile may be even more brazen than normal. The refugees say their attackers were not militias but uniformed Sudanese soldiers.

The latest round of fighting came after South Sudan split from the north last July to become an independent nation. Two states where people had fought with the southern Sudan People’s Liberation Army (SPLA)—South Kordofan and Blue Nile—were left on the north side of the border. Smarting from the loss of South Sudan, Bashir’s government declared it would tolerate no more dissent within its borders. But the rebels in South Kordofan and Blue Nile refused to disarm. War broke out in South Kordofan last June, then in Blue Nile in September. After bombers began attacking territory in Blue Nile, refugees poured out of rebel strongholds in both the Christian strip of southern Blue Nile and the mostly Muslim Ingassana Hills.

The latest refugees are almost all from Ingassana. And they report the humanitarian toll in that area is far worse than previously thought. For months, thousands were trapped in the mountains and prevented from fleeing to South Sudan by a Sudanese garrison blocking the route. “People were dying because of no food and water in the mountains,” said Jak Ka Tukul, from the Melbil area. “If a soldier found you looking for food or water, he’d shoot you.” Then, in May, the heaviest Sudanese offensive to date swept into the mountains, destroying those villages still standing. A rebel spokesman, Arnu Ngutulu, claims that by early June his forces had regained control of the area. But by then, as many as a quarter of the Ingassana population of 270,000 had fled to South Sudan.

The latest exodus of 40,000 refugees is in addition to some 70,000 who have already fled the entire Blue Nile state since last September, as well as 60,000 more who have crossed into South Sudan from South Kordofan. An already complicated situation is made more thorny by continued skirmishes between South Sudan and Sudan along their mutual but disputed border and a tense financial stand-off between the two sides over how to split revenues from South Sudan’s oil (which has to be exported through the north). That dispute prompted the South to cut off its oil supplies in January and plunge both countries into simultaneous budget crises. The revenue loss also prompted the Sudanese government to impose harsh austerity measures, which, in turn, led to a rising wave of student-led anti-government protests in Khartoum this month.

The U.S. has donated $30 million in emergency aid to the refugees and U.S. officials are gathering evidence of the human rights abuses alleged by the refugees. Yet Sudan is already sanctioned to the brim, and any sort of armed intervention does not appear to be on the table. U.S. officials say that want regime reform, not regime change in Khartoum. They fear any more instability in the Sudans will only increase the likelihood of an all-out state collapse in one or the other—which seems more and more feasible the longer South Sudan’s oil pumps sit idle.

On June 30, the anniversary of Bashir’s seizure of power in 1989, activists in Khartoum are planning their biggest-ever demonstration against the government. In the face of a crackdown by the government’s bloated security and intelligence services, they face an uphill battle. But with no peace talks on the agenda and no appetite for international intervention in Sudan, for refugees like Hassan Musa and Jak Ka Tukul, a revolution in Khartoum represents their best hope for being able to return to their destroyed homes anytime soon.